Temperature Dependence of the Electrical Resistivity of Polymerized C 60 Thin Films
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Temperature Dependence of the Electrical Resistivity of Polymerized C60 Thin Films R. Govinthasamy, J. H. Rhee, and S. C. Sharma* Department of Physics, The University of Texas at Arlington, Arlington, TX 76019, USA *
Author to whom correspondence should be addressed, [email protected]
ABSTRACT Highly conducting thin films of C60 were deposited by thermal evaporation in high vacuum on single crystal silicon substrates. The microstructure of the films was characterized by using Atomic Force Microscopy, and laser Raman spectroscopy. The films were polymerized by uv irradiation. The dc electrical resistivities of the as-deposited and uv-polymerized films were measured as functions of temperature between 295 and 17K by the four-probe technique. We present results on the effects of uv-irradiation on the surface microstructure and the temperature dependence of the electrical resistivity of these films. INTRODUCTION The electronic properties of C60 thin films continue to be of significant interest [1, 2]. The C60 molecule consists of a total of 60 equivalently bonded carbon atoms making a soccer-ball cage structure. Each carbon atom in this cage structure is bonded with three other carbon atoms through three σ and one π bonds; with intercarbon distances of 0.145 nm and 0.140 nm for single and double bonds, respectively. In crystalline C60, the molecules are bound by van der Waals forces in a closed packed face-centered-cubic (fcc) lattice with a lattice constant of 1.42 nm. The material undergoes a structural phase transition, from fcc to simple cubic structure, at Tc = 260 K. Above Tc, the molecules rotate around their lattice positions, whereas this motion is restricted below Tc; being frozen at 90 K. It has been reported that C60 can be polymerized above Tc by optical excitations as well as by the application of high pressure and high temperature [3, 4]. In the polymer phase, the adjacent molecules are covalently bonded by a four-membered ring joining two molecular cages. A photochemical [2 + 2] cycloaddition reaction has been proposed for the photoinduced dimerization in crystalline C60. Details of the photo-induced formation of the intermolecular covalent bonds, however, still remain the subject of discussion. Excellent reviews on the C60 based materials are available in the literature [1, 2, 5]. Here we present results from a series of experiments designed to further study the effects of uv-irradiation on the microstructure and the temperature dependence of the dc electrical resistivity of C60 thin films grown on single crystal silicon substrates. Some of the related results on our work on C60 based materials have been published elsewhere [6-8]. EXPERIMENTAL DETAILS C60 powder of 99.9% purity and containing 1-10 µ m particles was obtained from MER [9]. Thin films were deposited by thermal evaporation of C60 powder under high vacuum. A schematic of our high vacuum thin film growth system is shown in Figure 1. It consists of a
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vacuum chamber with base pressure of ~10-8 Torr. During thermal evaporatio
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